An XML document with correct syntax is called "Well Formed".
An XML document validated against a DTD is "Well Formed" and "Valid".
A "Valid" XML document is a "Well Formed" XML document, which also conforms to the rules of a DTD:
The DOCTYPE declaration, in the example above, is a reference to an external DTD file. The content of the file is shown in the paragraph below.
The purpose of a DTD is to define the structure of an XML document. It defines the structure with a list of legal elements:
The DTD above is interpreted like this:
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#PCDATA means parse-able text data. |
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A doctype declaration can also be used to define special characters and character strings, used in the document:
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An entity has three parts: an ampersand (&), an entity name, and a semicolon (;). |
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With a DTD, your XML files can carry a description of its own format.
With a DTD, independent groups of people can agree on a standard for interchanging data.
With a DTD, you can verify that the data you receive from the outside world is valid.
If you want to study DTD, you will find our DTD tutorial on our homepage.
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